The development of solid-state power amplifiers for RF transmitters has created challenges to designers not present when using previous tube designs. One major problem with solid-state designs is their limited power handling capability. While high power devices have been developed, they are generally quite expensive and thus are not desirable for designs where cost is a significant factor.
One strategy for solving this dilemma has been to divide the signal to be amplified into several components and to direct these components to a like number of smaller solid-state power amplifiers. The outputs of the power amplifiers are then combined to provide an output signal level which is comparable to or higher than the output signal which could have been obtained from a single high power solid-state power amplifier.
This divide-and-conquer strategy has its own drawbacks, however. The primary drawback was that previous signal dividers and combiners had used conventional wound transformers and lumped inductive and capacitive components to achieve the required impedance matching. Such components are inherently narrow-banded and are thus impractical for applications where wide bandwidths are required. Modern solid-state power amplifiers are generally broad-banded, and conventional narrow-banded signal dividers and combiners severely limited their utility.
One solution to such narrow-banded dividers and combiners was provided by U.S. Pat. No. 4,774,481 to Edwards et al., which discloses a broadband non-directional signal combiner (non-directional meaning that the combiner can be used as either a combiner or a divider). The combiner utilizes coaxial cables interconnected in a bridge configuration, and a coaxial cable transformer. The bridge configuration increases bandwidth, while the transformer counteracts the impedance transforming characteristics of the combiner. The resulting combiner disclosed by Edwards et al. combines and divides signals across a broad range of frequencies with relatively large isolation between input ports, and a low voltage standing wave ratio. However, the combiner disclosed in Edwards et al. is not entirely flux canceling when in the coherent mode. In addition, the combiner has a relatively large number of interconnections which act as discontinuities in the circuit, which increase insertion losses.
Another solution to the problems associated with narrow-banded dividers and combiners has been proposed in commonly assigned U.S. patent application Ser. No. 09/067,852. The combiner disclosed therein utilizes coaxial cables which are wound into coils. This arrangement provides a combiner having a relatively short signal path with few discontinuities, such that insertion losses are low and relatively little inductance is required in the signal path. However, this configuration may not be able to provide as high a bandwidth as may be desired, which may be up to 50:1 or higher for example.
What is still needed, therefore, is a non-directional signal combiner which exhibits exceptional power handling ability with low insertion loss characteristics, which exhibits excellent isolation characteristics between input ports, which exhibits excellent input and output port voltage standing wave ratio characteristics, which is capable of dissipating relatively large amounts of unbalanced power, which employs flux canceling circuitry combined with transmission line mode impedance matching which inherently exhibits excellent IMD characteristics, which exhibits a usable bandwidth of a decade or more and which is rugged and reliable, and of a relatively simple design that is conducive to relatively inexpensive mass production.